The invention relates more particularly to a pneumatic brake booster for a motor vehicle, of the type comprising a rigid casing inside which there can move a transverse partition sealingly delimiting a front chamber, subjected to a first pressure, and a rear chamber subjected to a second pressure that varies between the first pressure and a pressure higher than the first pressure, which is able to act on an actuating rod of a master cylinder associated with the booster by way of a reaction disk, and which is elastically returned toward its rest position by first return means, of the type which comprises a control rod moving in the casing selectively according to an axial input force exerted forward against the action of a return force exerted on the rod via second return means, of the type in which the movements of the control rod are able to determine the openings and closings of at least one axial intake valve which is inserted between a pressure source subjected to the pressure higher than the first pressure and the rear chamber, and of at least one axial equalizing valve which is inserted between the front chamber and the rear chamber, to actuate the moving partition, and of the type in which a plunger, passing through the moving partition and secured to the end of the control rod, is able to act directly upon the actuating rod of the master cylinder by way of the reaction disk.
Numerous examples of conventional boosters of this type are known.
In such a booster, the piston is secured to the moving wall. It is, for example, fitted through the moving partition. Furthermore, the equalizing valve and the intake valve form part of a single three-way valve a common seat of which is formed of a stepped front face of a moving element which is elastically returned against a flange of the plunger and against a flange of the piston from which flanges it can be selectively separated in order to open the equalizing valve or the intake valve.
Thus, a roughly radial duct which passes through the piston and which opens into the front chamber, on the one hand, and a roughly axial duct which opens to the outside of the booster, on the other, can be placed selectively in communication by way of a valve associated with a radial duct which passes through the piston and which opens into the rear chamber in order to maintain or cancel a pressure difference between the rear chambers and cause the moving partition to move.
Such a design has the disadvantage of entailing ducts made in the piston with complex shapes, which disturb the air flow. As a result, a booster generally has relatively high response times and proves noisy.
Furthermore, such a design is particularly expensive to implement because it entails the use of a piston whose interior shapes, intended to form the ducts, can be produced only by a machining method.